The most famous anonymous source of all: Robert Redford as Bob Woodward meets 'Deep Throat' in All The President's Men. Photograph: Ronald Grant Archive

It's the number one readers' grouse at the New York Times, an irritation showing alarming growth on a new computerised count of NYT usage monitored by researchers at the Poynter Institute. Yes, those "anonymous sources" are at it again, exclusively revealing this or that without – readers insist – giving them the information to judge what's real and significant: "the lifeblood and bane of journalism", as one Times ombudsman once put it. Which brings us, in the nature of the week, to Nick Davies and his Hack Attack saga of phone hacking.

The Guardian reporter who turned over all the Murdoch stones tells a brilliant story. Sir Harold Evans is right to call it an "exhilarating demonstration" of the best in journalism. Davies takes us through the years of sniffing and delving with energy, determination and resilience: and the Guardian itself, under bombardment from many directions, has much to be proud of, too. Investigative journalism perennially lives between triumph and disaster. It takes real guts – and a load of sources – to keep right on to the end of the road.

Carl Bernstein, of Washington Post Watergate renown, is another fan of these revelations about "the insidious abuse of power". And his involvement makes an interesting link – and commentary on the difference between daily filings and books. Woodward and Bernstein cherished the anonymous source Deep Throat once they put their investigations between hard covers. Davies, in similar mode, has Mr Apollo, Ovid, the Emissary, Sand, Jingle and a host of lesser informants who mostly don't wish to be named.

Obviously, he couldn't manage without them. Mr Apollo, indeed, is the fellow who sets the whole show rolling. He calls, out of the blue, and begins to produce evidence – drawn from the £1m settled and confidentially sealed case brought by Gordon Taylor, the footballers' union boss – that shows that Clive Goodman and Glenn Mulcaire, convicted hackers, weren't acting alone, as News International insisted, but as part of something much bigger. Dirty washing starts tumbling from sundry cupboards. And when Davies has problems fending off Wapping retaliation, just before a vital select committee hearing, a "supporter" of Apollo (let's call her Daphne to keep the myth-making going) suddenly surfaces to let Nick produce vital documents from the Taylor case before MPs.

Such anonymity has its continuing problems. Early on, the Guardian had a formidable range of leads but nobody who'd put a name to them. The gap between knowing something to be true and proving it yawned alarmingly. There's a constant tug of frustration that all great investigative reporters experience. Proof, proof, proof: testimony, details, documents.

Of course anonymity is a matter of reader trust, as an editor of the New York Times once insisted when he instituted rules that meant, among other things, at least one line editor being told the identity of every source before publication (whereupon spongy sourcing declined by 50%). But of course, too, there's that difference between the day-to-day and the whole package which poses different questions. Any curious reader of Hack Attack will inevitably want to know which government minister teed up The Emissary to help make Wapping's defences creak; why Ovid, with so many clues to his identity provided, can't now be named; and, particularly, who the deus ex machina called Mr Apollo is. (Although, between the lines, you can make a pretty good guess.)

For identity is also motivation, which may cover anything from moral disgust to commercial gain. Identity is part of the big picture. Davies performed a vital, continuing service when he revealed the true "sources" of hundreds of grubby News of the World yarns. Woodward and Bernstein signed off after we found out that Mark Felt was Deep Throat. Learning lessons means uncovering the whole truth, maybe in a second edition. The hunt for whistleblowing heroes as well as villains is part of journalism's lifeblood, too.